Fetal heart rate (FHR) detection is the primary methodology for antenatal determination of fetal well-being and assisting in the identification of potential hazards such as hypoxia and distress to the fetus. The expected outcome of early detection is a reduced risk of fetal morbidity and mortality.
Currently, FHR is most commonly detected using Doppler ultrasound where the standard predelivery test of fetal health is the fetal nonstress test (NST). These tests are routinely performed at the hospital with continuous-wave instruments.
Although current ultrasonic FHR detectors are becoming less expensive and less bulky, accurate sensor alignment and some degree of expertise are still required to correctly operate them. In addition, they are sensitive to motion and the safety of long-term fetal exposure to ultrasound waves has yet to be established. As a result, only short-term testing is actually practiced.
One alternative is the fetal electrocardiogram (FECG), but the procedure is more complex and less practical. In addition, commercial devices operating on noninvasive FECG are not available today.
More recently, optical methods that are still at the research stage have been proposed, in which halogen and tungsten lamps are used as light sources and a photomultiplier performs detection. However, these techniques are typically expensive, require high optical power, and are difficult to implement due to size and power consumption limitations.
An Optical FHR Detection System
A research team at the National University of Malaysia proposed a low-power optical technique based on the photoplethysmogram (PPG) signal, which is generated when a beam of light is modulated by blood pulsations, to noninvasively estimate the FHR. The doctor or technician shines a beam of LED light (less than 68 mW) at the maternal abdomen, modulated by the blood circulation of the mother and fetus. Maximum light wave penetration is achieved at a wavelength of 890 nm. This mixed signal can be processed by an adaptive filter using digital signal processing with the maternal index finger PPG as a reference input.
The team developed the optical FHR (OFHR) detection system using National Instruments’ LabVIEW graphical system design software and hardware. In the OFHR system, reducing the input power of the incident radiation led to a lower SNR, and the excitation signal was a chopped light beam. The system performed synchronous detection, and a software sub-routine generated the modulation frequency through a counter port using the NI 9474 digital output module.
At the receiver side, low-noise amplification and synchronous detection ensured conservation of the information with minimum noise power. A 24-bit NI USB-9239 analog-to-digital converter (ADC) minimized the effects of quantization noise. Once digitized, the signal was processed via adaptive noise canceling (ANC) techniques to extract the fetal PPG from the mixed signal.
The fetal probe (primary signal) was attached to the maternal abdomen using a Velcro belt to hold the IR-LED and photodetector separated by 4 cm, while the reference probe was attached to the mother’s index finger. Because the selected IR-LED could only emit a maximum optical power of 68 mW, the OFHR system operated with an optical power less than the limit of 87 mW specified by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). To modulate the IR-LED, the modulation signal was generated at a frequency of 725 Hz using the software subroutine through a NI 9474 counter port to the LED driver.
A low-noise (6 nV/Hz1/2) transimpedance amplifier converted the detected current to a voltage level. The reference probe (the mother’s index finger) consisted of an IR-LED and a solid-state photodiode with an integrated preamplifier. The signal from this probe was denoted as I(M2); M2 refers to the maternal contribution. Synchronous detection was not required at this channel because the finger photoplethysmogram had a high signal-to-noise ratio (SNR).
The NI USB-9239 24-bit resolution data acquisition module simultaneously digitized the detected signals from both probes at a rate of 5.5 kHz. The demodulation, digital filtering, and signal estimation were all performed in the digital domain. Software implementation consisted of generating a modulation signal, a synchronous detection algorithm, down-sampling, high-pass filtering, and an adaptive-noise-cancelling (ANC) algorithm.
The team used LabVIEW to implement the entire algorithm and part of the instrument. After preprocessing and applying the ANC algorithm, LabVIEW displayed results for the fetal signal and the FHR.